Environmentally friendly reduced graphene oxide as a broad-spectrum adsorbent for anionic and cationic dyes via π-π interactions

pi-pi interactions between graphene and organic dyes with a conjugate aromatic structure play a key role in the field of high-efficiency, broad-spectrum adsorbents for the removal of water contaminants. L-Cysteine reduced graphene oxide (RGO-Cys) has a good conjugate structure and dispersity in aqueous solution, endowing it with great adsorption efficiency towards anionic, nonionic and cationic dyes with a conjugate aromatic structure mainly via pi-pi interactions, as proved by the Raman spectrum and special adsorption experiments. The maximum adsorption capability for anionic indigo carmine (IC) and cationic neutral red (NR) is as high as 1005.7 mg g(-1) and 1301.8 mg g(-1), respectively, the former being the highest among those reported for adsorbents known to date. The total adsorption amount in mixed dye solutions is even higher (>3500 mg g(-1)), the highest total capability for simultaneous adsorption of anionic and cationic dyes in their solution mixtures. The pi-pi stacking adsorption mechanism ensures RGO-Cys to be used as a broad-spectrum adsorbent with high efficiency for many kinds of dye contaminants in water white the remnant carboxyl groups on graphene nanosheets facilitate effective adsorption towards Cu2+ with a capability as high as 139.2 mg g(-1), opening up many possibilities for the use of graphene in water cleaning including disinfection, decontamination, and desalination.